1. Field of the Invention
This invention relates to cochlear response instrumentation, and to improved techniques for stimulating, recording and processing cochlear responses (acoustic sounds emanating from the inner ear).
Cochlear response audiometers are used for the purpose of research into the physiology of the inner ear and for assistance in diagnosing and screening such conditions as noise-induced hearing loss and sensori-neural hearing loss in infants as well as adults.
2. Description of the Prior Art
Existing cochlear emission instrumentation and related problems can be described as follows.
The following comments are based on the use of Otodynamics instrument model ILO88, as well as information and the description in Peter John Bray's thesis "Click evoked otoacoustic emission and the development of a clinical otoacoustic hearing instrument", University College and Middlesex School of Medicine, London (UK), June 1989. The Abstract of the Disclosure of a United States patent relating to the instrument, U.S. Pat. No. 4,374,526 (David T. Kemp, granted May 17, 1983), states:
"A hearing faculty test and apparatus therefor is based on the finding that sound input to the ear gives rise to a returned wave from and related to the condition of the inner ear, this wave being detectable as an echo from the ear drum. The apparatus preferably comprises a sealing aural probe housing transducers respectively to project a repetitive transient sound by pulse generator activation and to pick-up for detection successive echoes by time-gating. Detected echoes are preferably averaged during processing for display. The echo occurs about 5-20 ms after its sound and a maximum operating frequency of about 50 Hz is appropriate. A continuous sound input can be used with consequent echo interference detectable as rapid changes of acoustic impedance with sound input frequency. Another alternative can involve detection of the ear drum movement by returned waves."
The ILO88 device, which is based on the above patent, uses 80 .mu.s clicks as stimulation. The peak Sound Pressure Level ("SPL") in the ear canal is 80-85 dB. These high levels, compared to the emission levels of 0-20 dB, create problems in the sensitive preamplifier as well as in the signal storage section. To compensate for the problem, the stimuli are presented in the form of click trains, each containing 4 stimuli, as shown in FIG. 1. The first 3 stimuli are positive with a certain amplitude (1.67 V), and the fourth is negative with an amplitude 3 times the amplitude of the positive-going stimuli (5.0 V). This way, the stimulus artefacts are partly cancelled, as well as some of the responses from the cochlea. For this reason, the first 4 ms of the response are not displayed. Another problem is that the ringing (i.e., the direct after-effect) from a brief 80 dB SPL signal in the ear canal could still be at a level where it interferes with the cochlear emission, especially at higher frequencies, where the delay of the emission is short (1-5 ms).
In the clinic, there are a few serious problems. One problem is that it is very difficult to fit the probe properly, especially in babies and infants. This is mainly due to the way the probe is designed, the availability of ear tips for a good seal, as well as the acoustic noise introduced into the probe when the probe cord is touched. If environmental noise is too high (e.g. due to air conditioning), then it is very difficult to perform a test. If the patient's hearing loss is higher than 20-25 dB, then no emission can be measured.